There is a need in the art for a photothermographic material for medical diagnostic and graphic arts use that has the ability to be efficiently, exposed by laser imagesetters or laser imagers and has the ability to form sharp black images of high resolution and sharpness. The goal is to eliminate the use of wet processing chemicals and to provide a simpler environmentally friendly thermal system to the customer.
Light sensitive recording materials may suffer from a phenomenon known as halation which causes degradation in the quality of the recorded image. Such degradation may occur when a fraction of the imaging light which strikes the photosensitive layer is not absorbed but passes through to the film base on which the photosensitive layer is coated. A portion of the light reaching the base may be reflected back to strike the photosensitive layer from the underside. Light thus reflected may, in some cases, contribute significantly to the total exposure of the photosensitive layer. Any particulate matter in the photosensitive element may cause light passing through the element to be scattered. Scattered light which is reflected from the film base will, on its second passage through the photosensitive layer, cause exposure over an area adjacent to the point of intended exposure. It is this effect which leads to image degradation. Photothermographic materials are prone to this form of image degradation since the photosensitive layers contain light scattering particles. The effect of light scatter on image quality is well documented and is described, for example, in T. H. James "The Theory of the Photographic Process", 4th Edition, Chapter 20, Macmillan 1977.
It is common practice to minimize the effects of light scatter by including a light absorbing layer within the photothermographic element. To be effective, the absorption of this layer must be at the same wavelengths as the sensitivity of the photosensitive layer. In the case of imaging materials coated on transparent base, a light absorbing layer is frequently coated on the reverse side of the base from the photosensitive layer. Such a coating, known as an "antihalation layer", effectively prevents reflection of any light which has passed through the photosensitive layer.
A similar effect may be achieved by a light absorbing layer interposed between the photosensitive layer and the base. This construction, described as an "antihalation underlayer" is applicable to photosensitive coatings on transparent or non-transparent bases. A light absorbing substance may be incorporated into the photosensitive layer itself, in order to absorb scattered light. Substances used for this purpose are known as "acutance dyes". It is also possible to improve image quality by coating a light absorbing layer above the photosensitive layer of a photographic element. Coatings of this kind, described in U.S. Pat. Nos. 4,581,323 and 4,312,941 prevent multiple reflections of scattered light between the internal surfaces of a photographic element.
Photothermographic antihalation systems for infrared materials have been described previously. However these usually had some disadvantages. A strippable antihalation coating of infrared absorbing pigment such as carbon black is described in U.S. Pat. Nos. 4,477,562 and 4,409,316. A strippable layer would generally have adhesion difficulties in processes such as coating, converting and packaging and also generates a sheet of pigmented waste material. For these reasons, it is not a desirable solution to the problem.
European Patent Application 0 377 961 and U.S. Pat. No. 4,581,325 describe infrared antihalation systems for photographic and photothermographic materials incorporating polymethine and holopolar dyes respectively. However, these dyes although having good infrared absorbance, have visible absorbance that is too high for use in subsequent exposures.
Antihalation systems that would satisfy the requirement of an IR/visible absorbance ratio of 30 to 1 would be the thermal-dye-bleach construction described in European Patent Application 0 403 157. The bleaching, infrared antihalation system uses a polymethine dye which is converted to a colorless derivative on heat processing. However, the system is not heat stable and as the dye decomposes,the IR absorbance decreases with time.
A second IR antihalation construction with a 30 to 1, IR/visible ratio can be prepared with indolenine dyes. Indolenine dyes have been described as IR antihalation dyes in silver halide, photographic materials in U.S. Pat. Nos. 2,895,955; 4,882,265; 4,876,181; 4,839,265 and 4,871,656 and Japanese Patent Kokai J63 195656. Infrared absorbing indolenine dyes have been described for electrophotography in U.S. Pat. No. 4,362,800, for optical laser recording material in Japanese Patent Kokai J6 2082-082A and J6 3033-477 and for photothermographic materials in Japanese Patent Kokai J4 182640.
In addition to proper antihalation, a critical step in attaining proper sensitometric properties is the addition of photosensitive silver halide. It is well known in the art that the addition of silver halide grains to a photothermographic formulation can be implemented in a number of ways but basically the silver halide is either made "ex situ" and added to the organic silver salt or made "in situ" by adding a halide salt to the organic silver salt. The addition of silver halide grains in photothermographic materials is described in Research Disclosure, June 1978, Item No. 17029. It is also claimed in the art that when silver halide is made "ex situ" one has the possibility to control the composition and size of the grains much more precisely so that one can impart more specific properties to the photothermographic element and can do so much more consistently than with the "in situ" technique.
Other performance characteristics influenced by the silver halide component and ones that are desired to achieve high quality photothermographic material for medical and graphic arts applications are; increased development efficiency, are desired to achieve high quality photothermographic materials for medical and graphic arts applications, are increased development efficiency, increased photographic speed, increased maximum density and lower Dmin and lower haze. U.S. Pat. No. 4,435,499 claims that these characteristics are not well addressed by conventionally prepared cubic grain silver halide gelatino photographic emulsions used in "ex situ" formulations. In fact, they claim advantages for tabular grains that give increased speed while maintaining a high surface area so that silver efficiency remains high. However it is well known that tabular grains give broad distributions which usually results in photosensitive materials of lower contrast than monomodal distributions. This is undesireable for our intended applications.
While the patent demonstrates increased speed and increased development efficiency, they do not show that increased Dmax is attained or that Dmin and haze remain lower than if very fine conventional cubic grains are used. In fact, it is known that larger grains tend to give high levels of haze.
Infrared supersensitization of photographic and photothermographic materials in order to attain increased sensitivity is described in detail in U.S. patent application Ser. No. 07/846,919 filed Apr. 13, 1992.